1. Field of the Invention
This invention relates to the in-situ retorting of carbonaceous deposits to produce fluid fuels, and more particularly to an improved method of producing such fuels in which danger of exposing workers to toxic gases is minimized.
2. Description of the Prior Art
Immense potential sources of carbon-containing compounds suitable as fluid fuels exist in subsurface carbonaceous deposits of oil shale, coal, and heavy, highly viscous petroleum oils. The highly viscous petroleum oil deposits are frequently referred to as tar sands. Because the carbonaceous material in the deposits is either solid as in oil shale and coal or highly viscous as in tar sands, treatment of the carbonaceous deposit to make the carbon-containing compounds fluid is necessary to deliver them from the deposit to the surface. A method of treatment that has been used is to heat the deposit to a temperature at which fluid carbon-containing compounds are formed or the viscosity of heavy oils is drastically reduced. One method of heating the deposit is by in-situ combustion in which a portion of the carboniferous material in the deposit is burned in place by igniting the deposit and injecting air into the deposit to heat oil shale or tar sands to a temperature at which oils of low viscosity are produced or to produce combustible gaseous products from coal.
The very low permeability of oil shale to the flow of fluids makes it necessary to rubblize the shale to form an in-situ retort through which fluids for heating the shale to a temperature high enough to convert the kerogen to shale oil can be circulated. While sometimes coal and tar sands may be sufficiently permeable for an in-situ combustion process, rubblization of those deposits can be advantageous in reducing channeling through the deposits. One of the methods of forming an in-situ retort is described in U.S. Pat. No. 1,919,636 of Karrick. In the process described in that patent, a vertical central shaft is driven through the oil shale to provide the desired void space necessary for permeability and the oil shale is blasted from the walls of the shaft to fill the shaft with broken oil shale. Other mining procedures for forming a rubblized in-situ retort are described in U.S. Pat. Nos. 2,481,051 of Uren, 3,001,776 of Van Poollen and 3,661,423 of Garrett. Those patents suggest using various mining techniques such as sublevel stoping, sublevel caving, block caving and shrinkage stoping to form the in-situ retort.
It is necessary during the in-situ retorting of oil shale to introduce the combustion air into the retort at a pressure substantially above atmospheric pressure to force the air through the retort at a rate high enough to maintain combustion of oil shale in the retort. The large volume of gases that must pass through the retort and the height of the retort cause a substantial pressure drop in the gases as they travel from the combustion air inlet preferably at the top of the retort to the outlet of the retort at its lower end. If it were attempted to maintain subatmospheric pressure in retorts during the burning of oil shale, the volume of the gases drawn from the retort would be excessive.
It is likely that there will be fractures in the oil shale formation that extend outwardly from rubblized retorts. The fractures will in many instances be naturally occurring fractures in the formation that existed before the retort was constructed but may also be fractures formed during the rubblization. Since retorts are maintained at substantially atmospheric pressure while being prepared for rubblization and during the rubblization, there is danger of flow of gases from a retort that is in the process of retorting oil shale to retorts that are being prepared for rubblization or are in the process of being rubblized. Gases developed during the retorting are highly toxic and may develop explosive mixtures when mixed with air.